The invention relates generally to the field of molten metal casting and to the field of filtration. Specifically, the invention relates to a filtering device for molten metal.
Wrought aluminum castings are used to produce high-grade stock for use in cans, as foil, litho sheets, memory disc substrates and aircraft components, for example. The casting process typically involves providing metal in a molten state using a furnace. The molten metal is then ultimately delivered to a casting mold or a secondary receiver for holding the molten metal at an elevated temperature for later casting.
It is desirable to remove any impurities or inclusions that may have been introduced into the molten metal at some stage during the casting process. Although sedimentation and flotation methods, which capitalize on the density differences between the molten metal and impurities, have been used, it is well recognized that filtration techniques are much more efficient than those methods.
Molten metal filtration is usually accomplished using filter elements formed of a porous refractory material. The molten metal is directed through the filter elements such that impurities are removed. Typically, inclusions that are above 5 microns in dimension are filtered from the molten metal in high-grade applications. Filtration can be performed by flowing molten metal through a filter box, which houses a filter assembly in such a way that the assembly may be removed and replaced as it becomes contaminated with the removed impurities. Similarly, in a trough filtration system, a plate filter is positioned as a type of trap in the trough. Molten metal, therefore, passes through the plate as gravity flows it through the trough.
Much attention has been given to the design of filter assemblies used in casting processes. It is desirable to minimize the footprint "floor space," and volume occupied by the filter assembly in order to provide sufficient casting space. On the other hand, it is desirable to maximize the filter surface area in order to minimize the velocity of molten metal through the filter itself. An optimized design must balance the factors of footprint, total flow and velocity through the filter. Because filters must be periodically replaced, their cost is a significant factor that affects the feasibility of using a particular filter construction. Of equal importance is the down-time of a casting line required when a filter must be replaced. Obviously, productivity can be increased by decreasing filter change time. There have thus been efforts to provide relatively low-cost filtering devices which achieve a requisite throughput while occupying as little space as possible.
It is known to provide a filter assembly in the form of a cartridge filter unit that is constructed of a plurality of tubular filter elements adhesively bonded between end plates. Such a cartridge is described in U.S. Pat. No. 5,126,047, the subject matter of which is incorporated herein by reference. The cartridge comprises a plurality of generally horizontally oriented cylindrical tubes that are connected at their ends to a pair of spaced, generally parallel, ceramic refractory end plates. One end plate is provided with holes that are in fluid communication with the hollow interiors of the tubes. The other end plate abuts the closed ends of the tubes. In use, the filter cartridge is disposed within a filter box into which molten metal flows. The molten metal is filtered as it travels from the exterior of the tubes to the interior of the tubes. The filtered molten metal then flows through the tube interior and through the holes in the end plate to the outlet of the filter box. An advantage of a cartridge filter over a plate filter is that a cartridge filter provides a greater surface area within the same size footprint as a two-dimensional plate filter.
While such a filter cartridge construction offers superior filtration capabilities as compared to other filter constructions, its high cost may make it uneconomical. One of the primary reasons for the high cost associated with such prior art filter cartridges relate to their unitary construction. Specifically, the filter cartridge must be replaced as a single unit because filter elements are permanently bonded to the end plates using a refractory cement.
More importantly, this construction also results in considerable down-time during filter changes, since the entire cartridge must be preheated before use, typically requiring up to 24 hours for a cartridge with a large number of filter tubes. Another disadvantage is that the filter cartridge is generally a bulky structure which is difficult to lift and which must be carefully installed, often requiring significant manpower. To this end, holes are usually provided in the end plate to permit lifting equipment to attach to the end plate.